Well Water Contamination from Septic: Signs and Testing

Homes with private wells and private septic systems can face a risk that city-water homes don’t: septic system effluent contaminating the drinking water supply. This is most likely when the well and septic system are too close together or when the septic system is failing. Here’s what Washington homeowners need to know about this risk, the signs of contamination, and how to test.

How Septic Systems Can Contaminate Wells

Effluent from the septic system moves through the soil.

The drain field disperses partially treated wastewater (effluent) into the soil. Normally, soil provides additional treatment — filtering pathogens and breaking down contaminants before the effluent reaches groundwater or adjacent wells.

When this fails:
– Inadequate separation: The well and drain field are too close — effluent travels through the soil to the well before adequate treatment occurs
– Failing drain field: A failed drain field allows poorly treated effluent to surface or move through the soil without sufficient treatment
– Overloaded system: Excess effluent volume overwhelms soil treatment capacity
– High water table: Shallow groundwater reduces the distance between drain field and groundwater, allowing contaminants to enter the water table

What contaminants reach the well:
– Bacteria: E. coli, coliform bacteria, enterococcus — indicators of fecal contamination
– Nitrates: from the breakdown of organic nitrogen in sewage — particularly dangerous for infants
– Viruses: less common but possible in systems with very short travel distances to the well
– Pharmaceuticals and other emerging contaminants: from medications flushed or excreted

Washington Setback Requirements

Washington State regulations specify minimum distances between septic components and wells.

Under Washington’s On-Site Sewage System (OSS) rules:
– Minimum separation: 100 feet between a septic tank or drain field and a drinking water well
– For mound systems and alternative systems: varies by design and county regulations
– Some counties have stricter requirements depending on soil type and local groundwater conditions

This is a minimum, not a guarantee:
100 feet of soil separation provides substantial but not absolute protection. Soil type matters — sandy or gravelly soils provide less filtration than clay-rich soils. Groundwater flow direction matters — a well downhill from a drain field has more risk than a well uphill.

Existing non-conforming systems:
Many older Washington properties were developed before current setback requirements. Properties built decades ago may have wells and septic systems closer than 100 feet. These properties have elevated contamination risk and should be tested more frequently.

Signs of Well Water Contamination from Septic

Important: contaminated water often looks, smells, and tastes normal.

E. coli and coliform bacteria cannot be detected by sight, smell, or taste in most cases. The only way to know is to test.

Signs that may indicate contamination (but are not definitive):

• Sewage smell in well water: Hydrogen sulfide can indicate bacterial activity or nearby sewage. Other odors that are unusual or have appeared recently.
• Gastrointestinal illness in household members: Recurring nausea, diarrhea, or stomach illness without other explanation.
• Known septic system problems: Drain field failure, surfacing effluent, or recent sewage backup are times to test the well immediately.
• After flooding: Floodwaters that cover the wellhead can directly contaminate the well.
• Change in water clarity: Turbid or discolored water may indicate surface water infiltration.

None of these are diagnostic — test regardless.

How to Test Well Water for Septic Contamination

Testing is the only reliable method.

What to test for:

Total coliform and E. coli:
The standard test for fecal contamination — bacteria that indicate human or animal waste contamination. This is the first test to run.

Nitrates:
High nitrate levels indicate sewage or fertilizer contamination. The EPA maximum contaminant level (MCL) for nitrates is 10 mg/L as nitrogen. Levels above this are dangerous for infants.

Full potability panel:
For a comprehensive assessment, a full potability panel tests for additional bacteria, nitrates, turbidity, pH, hardness, and other parameters. Recommended for baseline testing when you first move in or when you haven’t tested in several years.

Where to get tested:
Use a state-certified water testing laboratory. Washington State Department of Health maintains a list of certified labs. Do not rely on water softener or filter company “free tests” — these are sales tools, not independent assessments.

Sampling correctly:
Follow the lab’s instructions for sampling — typically:
– Use the sample container provided by the lab
– Flush the cold water tap for 2 minutes before sampling
– Take the sample from an unfiltered, non-softened tap (before any treatment)
– Keep the sample cold and deliver to the lab within the time window (typically same day or overnight)

Cost:
– Bacteria (coliform + E. coli): $25–$75
– Nitrates: $15–$40
– Basic potability panel: $75–$150
– Comprehensive panel: $200–$400

Testing Frequency

Annual testing minimum:
Washington Department of Health recommends testing well water at least annually for bacteria. Homes with known septic systems nearby should test every year.

Test immediately if:
– A septic backup has occurred
– The drain field has been diagnosed as failing
– Household members experience unexplained gastrointestinal illness
– The wellhead was flooded
– Any construction activity near the well or septic system occurred
– Water appearance, smell, or taste has changed

What to Do If the Well Tests Positive for Contamination

Don’t drink the water — switch to bottled water or a known safe source immediately.

Step 1: Identify the source
Confirm whether the contamination is from the septic system. Have a septic professional assess the system for failure. If the system is functioning properly, contamination may have another source (surface runoff, animal waste, nearby agricultural operations).

Step 2: Address the septic system
If septic failure is confirmed — drain field failure, system overloading, inadequate separation — repair or replacement is needed. This is not optional when the well is being contaminated.

Step 3: Shock chlorinate the well
After the contamination source is addressed, the well can be shock chlorinated to kill bacteria present in the well and casing. This is a standard remediation procedure — instructions are available from Washington Department of Health.

Step 4: Retest
After shock chlorination, wait the specified time and retest. Continue to use alternative water sources until the well tests clean.

Step 5: Install point-of-entry treatment (if needed)
If separation distance cannot be corrected and the system cannot be relocated, point-of-entry treatment (UV disinfection, reverse osmosis) provides an ongoing treatment layer. This is a backup measure, not a substitute for a properly functioning septic system.

FAQ

Q: Can a septic system contaminate my well water?
A: Yes — if the well and drain field are too close, if the septic system is failing, or if the soil treatment capacity is overwhelmed, effluent can reach the groundwater and contaminate the well. Regular testing is the only way to know.

Q: How far should a septic system be from a well?
A: Washington State requires a minimum 100-foot separation between septic components (tank, drain field) and a drinking water well. Some counties have stricter requirements. Older properties may not meet current setback requirements.

Q: How do I test my well water for septic contamination?
A: Use a state-certified water testing laboratory. Test for total coliform and E. coli (bacteria test) and nitrates at minimum. Follow the lab’s sampling instructions — flush the tap before sampling, use the lab’s container, keep cold, deliver promptly. Results take 3–7 days.

Q: What contaminants from septic get into well water?
A: Bacteria (E. coli, coliforms, enterococcus), nitrates from organic nitrogen breakdown, and potentially viruses in systems with very short travel distances. Contaminated water often looks and tastes normal — testing is required to detect contamination.

Q: Is well water with septic contamination safe to drink temporarily?
A: No — do not drink water that has tested positive for E. coli or other sewage indicators. Switch to bottled water or a safe alternative immediately. Address the contamination source (septic system failure), shock chlorinate the well, and retest before returning to using the well.